In terms of easing your pain, ServiceNow are aware of this and there are roadmap items to improve this experience. Yokohama introduces an upgrade workspace as a starting point which may aid in your visibility. 

If you do a family upgrade (i.e moving from Xanadu to Yokohama) apps and plugins will "true-up" to a compatible version. However, this doesn't mean the latest version of that app. Individual product teams determine which version is aligned to a product family release. 

In terms of approach - focus on functionality your stakeholders desire. Those apps should be what you prioritise to upgrade (past that of the true-up version). An example would be SOW and Workflow Studio. The new features in those are often highly desirable and adopted on a quarterly basis